1. Field of the Invention
This invention relates to a method for locating concealed or buried pipe using the "time-of-flight" of pulsed acoustic signals injected into the pipe. The signal, generated in a variety of ways, propagates through the interior of the pipe and is detected by an array of detectors which, in turn generate signals which are processed in a mathematical algorithm to determine the location of the pipe.
2. Description of Prior Art
Reasons for being able to locate buried pipe are numerous and include avoidance of such pipe during excavation projects, attachment of various equipment to the pipe, leak detection and tapping directly into the pipe, especially while it is in service. Numerous methods exist for locating buried metallic pipe which, due to its electrical conductivity, lends itself to processes in which an electromagnetic signal is impressed onto the pipe and the strength of the magnetic field generated by the signal as measured at the surface of the ground is used to determine the location of the pipe. Similar techniques have been applied to plastic pipes which themselves are not electrically conductive but which have been buried with a "tracer wire" alongside, above or near them upon which an electromagnetic signal can be impressed. However, these techniques cannot be applied if the "tracer wire" breaks or otherwise corrodes. Consequently, alternative approaches which do not depend on the material properties of the pipes are required.
U.S. Pat. No. 3,967,282 and U.S. Pat. No. 4,062,010 disclose a process and apparatus for detecting underground pipelines using radar-type signals. U.S. Pat. No. 4,600,356 discloses a pulsed eddy current proximity process and apparatus combined with an excavation device for altering the operator of the device to underground obstructions. U.S. Pat. No. 4,649,948 discloses an apparatus and method for installing a Wye device within a pressurized pipe for inserting a probe into the pressurized pipe for detecting leaks in underground pipelines.
Another approach is the use of acoustics. Applications of acoustical signals to buried pipe are well known to those skilled in the art. Methods for detecting leaks in underground pipelines using acoustic signals are taught by U.S. Pat. No. 3,055,209; U.S. Pat. No. 3,170,152; U.S. Pat. No. 3,264,864; and U.S. Pat. No. 4,013,905. Generally, these methods involve generation of an acoustic signal at a point within a pipeline, detecting the signal at another point in the pipeline and analyzing the differences in the signal between the point at which it is generated and the point at which it is detected to determine the location of the leak. U.S. Pat. No. 4,172,382 discloses a method and apparatus for detecting leaks in buried pipelines by injecting an acoustic signal into the pipeline and, using a laser interferometer system, measuring the time varying displacements of the earth's surface which are propagated at the site of the leak. U.S. Pat. No. 4,858,462 discloses a method and apparatus for locating a leak in a medium such as a steam, gas or liquid pipeline. The apparatus requires a pair of spaced apart piezoelectric transducers which are mechanically coupled to the surface of the pipeline and between which is located a leak. A pressurized leak in the pipeline generates acoustic emissions which propagate along the surface of the pipeline. The emissions are detected by the piezoelectric transducers. By measuring the differences in arrival times of spikes in the acoustic emission at the piezoelectric transducers, the location of the leak can be determined. U.S. Pat. No. 4,457,163 discloses a method and apparatus for locating pipeline damage and medium leak therefrom by acoustically monitoring the soil above the pipe and determining the location of the loudest signal. Japanese Patent 0238733 discloses a method of determining water leakage in an underground water pipe by measuring the phase difference between a sound wave introduced through a branched pipe of a ground level branch of an underground water pipe and the sound wave received at a remote location on the surface of the earth.
The use of acoustic signals to detect the location of underground pipes is also known to those skilled in the art. U.S. Pat. No. 4,911,012 discloses a system for detecting the location of an underground sewer by introducing an identifiable, for example, pulsed acoustic signal into the sewer line at an accessible location and transmitting the signal through the sewer along the underground portion of the sewer line and sensing the signal from the surface at a remote location. The system utilizes a single detector and relies merely on sensing the amplitude of the acoustic signal to determine the sewer line location. Measurement of the amplitude of the acoustic signal is suitable for determining the general location, that is, within a few feet, of the underground pipe. However, it is not suitable for precise location, that is, within a few inches, of the underground pipe as required by most pipe location applications due to complex acoustic ground boundary effects which can reduce the amplitude of an acoustic signal at ground locations above the underground pipe. In addition, coupling of the detector to the ground will vary from placement to placement thereby introducing another uncertainty into the pipe location process, namely, the effect of the variances in coupling on the measured amplitude of the acoustic signal.